Issue link: https://iconnect007.uberflip.com/i/1181966
94 SMT007 MAGAZINE I NOVEMBER 2019 • For void% < 25%, when solder dot paste stencil is used, the presence or absence of solder mask doughnut is indifferent; the same number of components show <25% voiding. Also, there are no cases of void%>50%; the largest void size is 44% for the nine boards assembled with the dot solder paste deposit stencil • For void% < 25%, when segmented sol- der paste stencil is used, the absence of solder mask doughnut yields at least twice as many components with <25% voiding Post assembly, it was observed that sol- der paste wicked in the vias, and some pro- truded on the back side of the board. An SPI program was developed to measure the sol- der paste protrusions height. The results of the post assembly SPI inspection, shown in Figure 18, were analyzed based on the main interac- tions, similar to the void% main interactions analysis. In terms of significance, all the factors consid- ered for the main effects interactions are calcu- lated to be significant (except for solder mask), including some of the second-order interac- tions. The average protrusion height decreases with decreased via size. Void% and protrusion height responses change in the same direction with the number of vias, via pitch, board thick- ness, and solder paste coverage. They differ for solder mask, via size, and stencil type. How- ever, when plotted against each other, there is no correlation found between the void% and the height of the solder protrusions. Additional X-ray inspection was done for one board only (PCA#6, 110 mils, assembled with the segmented aperture stencil). Selected images are included in Figures 19–24 to illus- trate the presence of solder protrusions for different size vias and the wicking of solder inside the vias. There are a significant number of cases with no visible solder trapped in vias, like most of Figure 18: Main effects of solder mask (sm), via array, via size, via pitch, stencil type (dot=0, segmented=1), board thickness, and solder paste coverage on the solder paste protrusion height. Figure 19: Segmented stencil, via diameter f1 = 0.2 mm. Figure 20: Segmented stencil, via diameter f1 = 0.2 mm, solder mask doughnut around via. Figure 21: Segmented stencil, via diameter f2 = 0.23 mm.